1. Field of the Invention
The present invention relates to an achromatic lens system.
2. Description of the Related Art
To constitute the so-called apochromatic lens system in which axial chromatic aberrations at three wavelengths of light are well corrected, it is necessary to appropriately set up a functional combination of a plurality of lenses including an anomalous dispersion glass. However, anomalous dispersion glass is expensive and tends to be influenced by a variation of temperature.
An apochromatic lens system using no anomalous dispersion glass has been proposed, for example, in the Japanese Unexamined Patent Publication No. 5-142468 and No. 7-151964, in which a cemented lens consisting of positive and negative lens elements having different relative partial dispersions is provided at an intermediate location between a front lens group and a rear lens group. However, employing a cemented lens consisting of two lens elements adds to the manufacturing cost of the apochromatic system as a whole.
It is an object of the present invention to provide a simple and highly effective and efficient achromatic lens system (achromatic lens system) in which axial chromatic aberrations at three or more wavelengths can be effectively corrected.